diff --git a/src/parser.js b/src/parser.js
index 97d186e7..352a1d45 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -878,7 +878,12 @@ const parser = (() => {
         // elvis/default operator
         infix("?:", operators['?:'], function (left) {
             this.type = 'condition';
-            this.condition = left;
+            // Deep-clone `left` so the condition and then branches have
+            // independent AST nodes. Sharing the same reference causes
+            // post-parse processing (e.g. predicate stages, unary minus
+            // folding on number literals) to mutate the same node twice,
+            // producing wrong results (see #773 for the equivalent ?? fix).
+            this.condition = JSON.parse(JSON.stringify(left));
             this.then = left;
             this.else = expression(0);
             return this;
diff --git a/test/test-suite/groups/default-operator/case014.json b/test/test-suite/groups/default-operator/case014.json
new file mode 100644
index 00000000..0719d339
--- /dev/null
+++ b/test/test-suite/groups/default-operator/case014.json
@@ -0,0 +1,7 @@
+{
+    "description": "array predicate on lhs exists, should return lhs value",
+    "expr": "foo.blah[0] ?: 42",
+    "dataset": "dataset0",
+    "bindings": {},
+    "result": {"baz": {"fud": "hello"}}
+}
diff --git a/test/test-suite/groups/default-operator/case015.json b/test/test-suite/groups/default-operator/case015.json
new file mode 100644
index 00000000..7009e5c0
--- /dev/null
+++ b/test/test-suite/groups/default-operator/case015.json
@@ -0,0 +1,7 @@
+{
+    "description": "array predicate on lhs does not exist, should return rhs value",
+    "expr": "foo.blah[5] ?: 42",
+    "dataset": "dataset0",
+    "bindings": {},
+    "result": 42
+}
diff --git a/test/test-suite/groups/default-operator/case016.json b/test/test-suite/groups/default-operator/case016.json
new file mode 100644
index 00000000..c8676686
--- /dev/null
+++ b/test/test-suite/groups/default-operator/case016.json
@@ -0,0 +1,7 @@
+{
+    "description": "negative array index on lhs returns last element",
+    "expr": "[1,2,3][-1] ?: 42",
+    "dataset": null,
+    "bindings": {},
+    "result": 3
+}
diff --git a/test/test-suite/groups/default-operator/case017.json b/test/test-suite/groups/default-operator/case017.json
new file mode 100644
index 00000000..98332d3f
--- /dev/null
+++ b/test/test-suite/groups/default-operator/case017.json
@@ -0,0 +1,7 @@
+{
+    "description": "negative array index on single-element array returns the element",
+    "expr": "[true][-1] ?: \"no\"",
+    "dataset": null,
+    "bindings": {},
+    "result": true
+}
diff --git a/test/test-suite/groups/default-operator/case018.json b/test/test-suite/groups/default-operator/case018.json
new file mode 100644
index 00000000..f3a81072
--- /dev/null
+++ b/test/test-suite/groups/default-operator/case018.json
@@ -0,0 +1,7 @@
+{
+    "description": "unary minus on number literal LHS is not double-negated",
+    "expr": "-5 ?: 99",
+    "dataset": null,
+    "bindings": {},
+    "result": -5
+}